The cryogenic rectification of air to produce oxygen, nitrogen and/or argon is a well established industrial process. Typically the feed air is separated into nitrogen and oxygen in a double column system wherein nitrogen top vapor from a higher pressure column is used to reboil oxygen-rich bottom liquid in a lower pressure column. Argon-containing fluid from the lower pressure column is passed into an argon side arm column for the production of argon product.
Refrigeration for the system is typically produced by turboexpanding a portion of the feed air stream. The non-turboexpanded portion of the feed air is passed into the higher pressure column while the turboexpanded portion of the feed air is passed into the lower pressure column. The feed air is separated in the higher pressure column into nitrogen-richer and oxygen-richer components which are passed into the lower pressure column for final separation.
To increase production or performance of the plant, the refrigeration requirement may be increased, necessitating the turboexpansion of a larger fraction of the feed air. This increases the amount of feed air passed into lower pressure column. However, this reduces the separation efficiency of the lower pressure column and in particular tends to reduce the argon concentration in the argon-containing fluid passed from the lower pressure column into the argon column. This burdens the recovery of argon product from the argon column.
Accordingly, it is an object of this invention to provide an improved cryogenic rectification system which can provide enhanced argon recovery.
It is another object of this invention to provide an improved cryogenic rectification system wherein increased refrigeration may be produced without increasing the feed air fraction which is turboexpanded and passed into the lower pressure column.